Co-ordinated Sciences Chemistry C3: Stoichiometry

Chemistry C3 of Cambridge IGCSE Co-ordinated Sciences 0654 turns chemical symbols into quantitative predictions. Official sections C3.1 to C3.3 move from formulae and balanced equations to relative mass, simple reacting-mass proportions, concentration in g/dm³, moles, particles, limiting reactants and gas volumes at r.t.p. Titration calculations, empirical formulae, percentage yield and percentage purity are outside this boundary.

Formulae name particles and proportions

A molecular formula gives the number and type of each atom in one molecule. For example, one molecule of ethene, C₂H₄, contains two carbon atoms and four hydrogen atoms.

Ionic compounds form giant lattices rather than molecules. Their formula gives the simplest charge-balanced ratio of ions. Magnesium ions are Mg²⁺ and chloride ions are Cl⁻, so magnesium chloride is MgCl₂. Total positive and negative charge must cancel.

A model can also reveal a formula. Count the represented atoms or ions and distinguish a finite molecule from a repeating lattice.

Write chemical equations

A word equation names reactants and products. A symbol equation replaces names with correct formulae. Balance a symbol equation by changing coefficients only. Never alter a formula to make atom counts match because that changes the substance.

For magnesium burning:

magnesium + oxygen → magnesium oxide

2Mg(s) + O₂(g) → 2MgO(s)

State symbols are (s), (l), (g) and (aq). They describe physical state or aqueous solution under the reaction conditions.

An ionic equation removes spectator ions that remain unchanged. For precipitation of silver chloride:

Ag⁺(aq) + Cl⁻(aq) → AgCl(s)

Check both atom count and total charge. A valid ionic equation balances each.

Relative atomic, molecular and formula mass

Relative atomic mass, Ar, is the average mass of the isotopes of an element compared with one twelfth of the mass of a carbon-12 atom.

Relative molecular mass, Mr, is the sum of the relative atomic masses in a molecule. The same symbol is used for relative formula mass of an ionic compound.

For CaCO₃, using Ca = 40, C = 12 and O = 16:

Mr = 40 + 12 + (3 × 16) = 100

Ar and Mr are relative values without units. Molar mass is the mass of one mole and is written in g/mol. It has the same numerical value as the relevant Ar or Mr.

Core reacting-mass questions may use simple proportions without the mole concept. If 12 g of carbon reacts completely with 32 g of oxygen to form 44 g of carbon dioxide, 6 g of carbon forms 22 g of carbon dioxide because every mass is halved.